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Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells
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Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells Peng Zhang a,b, Xian-Hua Liu a,*, Ke-Xun Li b,**, Yi-Ren Lu a a
Tianjin Key Lab. of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072, PR China b College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, PR China
article info
abstract
Article history:
Petrocoke carbon (PC) was prepared by pyrolysis of easily available byproduct of oil
Received 1 May 2015
refining, petroleum coke. Thus-obtained PC not only possesses highly porous surface area,
Received in revised form
but also contains heteroatoms such as phosphate and sulfur in its framework. These
12 July 2015
properties make petrocoke carbon a good choice as a precursor for high performance air-
Accepted 12 August 2015
cathode catalyst. Microbial fuel cells (MFCs) with PC as air-cathode catalyst produced a
Available online xxx
maximum power density of 1029.77 ± 99.53 mW m2, which was higher than those with Pt/ C cathodes or activated carbon cathodes. Furthermore, PC cathodes have comparable
Keywords:
durability to activated carbon cathodes, demonstrating that the PC can be a low-cost and
Petroleum coke
reliable alternative to precious-metal-based electrocatalysts for the air cathode in MFCs.
Air cathode
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst MFCs
Introduction In the last decade, the application of microbial fuel cells (MFCs) has made tremendous progress and the MFC performance has increased remarkably [1e5]. For the scale up of MFCs, air cathode is the most useful configuration because of the inexhaustible availability of air and high redox potential of oxygen reduction [6]. During operation, the air passes through the cathode to the interior cathode active surface in contact with the cell's electrolyte. Catalysts are needed to enhance the oxygen reduction reaction (ORR) at the cathode. To date, platinum (Pt) has been the most used cathode catalyst in fuel cells. However, Pt based catalysts have a high cost and are easily poisoned by intermediates, which critically impeded
the extensive application of MFCs [7]. Recently, carbonaceous materials such as carbon nanotubes, graphene, and activated carbon (AC) had been found to be effective for the ORR [8e14] and were widely used to replace the precious metal catalysts. The excellent performance of carbonaceous materials was deemed to its large surface area, which reduced the overpotential with small local current densities [15], excellent ORR electrocatalytic performance, and long-term durability [10]. Meanwhile, the lower price of the carbonaceous materials enhanced the application of MFC technology in practice. Many kinds of carbonaceous materials had been used as MFC cathode material and showed different rates of performance [14,16e20]. Activated carbon fiber felt cathode has been reported being applied in an upflow MFC and a maximum power density of 784 mW m2 was achieved [21]. Carbonaceous
* Corresponding author. Tel./fax: þ86 22 27402367. ** Corresponding author. Tel./fax: þ86 22 23495200. E-mail addresses: [email protected] (X.-H. Liu), [email protected] (K.-X. Li). http://dx.doi.org/10.1016/j.ijhydene.2015.08.025 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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materials doped with heteroatoms such as nitrogen, sulfur, phosphorus, and boron have also been investigated because of their outstanding ORR catalytic performances [10]. Fox example, nitrogen-doped nanosheet on graphene was applied as the cathode catalyst in MFC and a peak power density of 1159.34 mW m2 was obtained [22]. However, most synthetic approaches of these carbonaceous materials are usually complex and costly, which limited their application in fuel cells. Petroleum coke is a waste byproduct of oil refining and is very abundant and cheap [23]. It is a black solid material consisting of polycyclic aromatic hydrocarbons with low hydrogen content and usually composed of C: 91e99.5%, H: 0.035e4%, S: 0.5e8%, (N þ O): 1.3e3.8% and the rest is metals [24]. Effort has been directed towards petroleum coke utilization in the fields of combustion and gasification to generate electric power or produce syngas. However, the applications for petroleum coke have been limited due to the high sulfur content (~6 wt%) and low surface area (<5 m2 g1). It had been reported that the activated petroleum coke can have a greater specific area than the ordinary activated carbon [25]. Due to its lower cost and higher specific area, PC may be considered as a high-performance and cost-effective cathode material. Until now, there is still little information available on its application in MFCs. In this study, we present an economical and environmental route to synthesize carbonaceous materials with highly porous surface area and active sites using a simple KOH activation process. These carbonaceous materials were utilized to synthesize air cathodes by a rolling method [9]. The performance of these materials as air-cathode catalyst for oxygen reduction reaction was evaluated in electrochemical tests and MFC systems. In addition, the surface properties of the PC were examined by scanning electron microscopy (SEM), BrunauereEmmetteTeller analysis (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). Furthermore, we examined the stability of these PC cathodes and compared them to normal AC cathodes after 1 month of continuous operation.
Materials and methods Preparation of PC cathode Petroleum coke from SINOPEC Tianjin Company was used as raw material and smashed by wet ball mill and dried in an oven at 80 C for 12 h. Then the petroleum coke was mixed with KOH with a weight ratio of 1:4 (petroleum coke: KOH). KOH is one of the typical alkaline metal compounds that is widely used for the chemical activation of coal precursor, chars and petroleum coke [26]. The ratio of petroleum coke to KOH plays an important role in the development of carbon porosity. According to D. Lozano-Castello [27], surface area and micropore volume reach the maximum with KOH/C ratio of 4:1. The mixture was ground with a mortar to ensure that the compositions could be well distributed, and it was activated in a pipe furnace in nitrogen atmosphere (purity: 99.99%) at 800 C. The activation process was that: the
temperature was increased to 300 C at a speed of 2 C min1 from room temperature and kept at this temperature for 1 h, then the temperature increased to the activation temperature (800 C) at a speed of 5 C min1 and this activation process lasted for 1 h. The activation product was cooled in nitrogen and washed with distilled water until the pH approached 7. Then the product was dried in the oven at 120 C for 1 day. The product was ground again to form a state of powder, which was used in the construction of PC cathode. The PC cathode was constructed with the same procedure as AC cathode described by Dong [9]: the PC was mixed with polytetrafluoroethylene (PTFE) and rolled on the water facing surface of stainless steel mesh with the mixture of carbon black and PTFE sintered on the air facing side. The normal AC (2132.95 m2 g1, Yihuan Carbon Co. Ltd., Fujian, China) was also used as the catalyst to construct AC cathode by the same method.
MFC setup and operation Single-chamber MFCs (40-mm long cylindrical chamber; volume 28 mL) were constructed as previously described [28]. Anodes were round-shape carbon felts (diameter 30 mm; thickness 5 mm). Both the anode and cathode had the same projected area of 7 cm2 and were connected to an external resistance of 1000 U using titanium wires. To ensure the same experimental conditions, the different cathodes (PC and AC) were constructed in the same MFC devices. The MFCs were inoculated using wastewater from the Jizhuangzi municipal sewage treatment plant (Tianjin, China) and had been cultivated for 2 months at 30 C in a biochemical incubator. The medium was 1 g L1 sodium acetate with vitamins and minerals dissolved in phosphate buffer solution (PBS) [29].
Measurements and analysis A multi-channel voltage logger (MPS-10, Morpheus Electronic Co. Ltd, Beijing, China) was used to record the voltages. To obtain polarization curves and power density curves, a variable resistance box (Tianjin, China) was used to vary the external resistance from 9000 U to 50 U. The voltage was recorded after a fixed time (about 30 min) to ensure a stable state. Cathode and anode potentials during polarization were measured using an Ag/AgCl electrode as the reference electrode. Tests were conducted in the first week and after 1 month of operation. All experiments were conducted in triplicate. Linear sweep voltammetry (LSV) of all the cathodes were measured at a scan rate of 0.1 mV s1 from open circuit potential (OCP) to 0.3 V by a CHI 660 potentiostat (Shanghai, China). An Ag/AgCl electrode was used as reference electrode and platinum sheet (1 cm2) was used as counter electrode. Electrochemical impedance spectroscopy (EIS) of all the cathodes was performed over a frequency range of 100 kHze100 mHz using a potentiostat (VersaSTAT 3, Princeton Applied Research, USA). An FTIR spectrometer (FTS6000) was utilized to detect the IR-observable functional groups on the surface of the PC and normal AC in the wavenumber range of 400e4000 cm1. Small amounts of samples were mixed with KBr and pressed into
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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pellets, which contains about 0.5% sample in weight [26]. To characterize the crystal structure of the catalysts, a Rigaku D/ Max-2500 V X-ray diffractometer was used to measure the Xray intensity over a diffraction 2-theta angle from 5 to 80. The dimension of carbon crystallites was analyzed by X-ray diffraction line broadening. The layer dimension D perpendicular to the basal plane was calculated by the (002) reflection. XPS (Axis Ultra DLD, Kratos Analytical Ltd.) was employed to obtain information about the elemental composition of the PC and AC samples. CasaXPS software was used to process the XPS data for the elemental analysis. The morphology of the PC and AC surface were examined by SEM (Hitachi S-3500N, Tokyo, Japan) at an accelerating voltage of 20 kV. The BET surface area of the catalyst layer was measured on an ASAP 2020/TriStar 3000 (Micromeritics) using nitrogen as adsorbents.
Results and discussion Surface morphology of PC Fig. 1 shows the surface morphology of raw petroleum coke, PC, and AC. The surface of raw petroleum coke (Fig. 1a) was smooth and wrinkled. After activation, the surface of PC (Fig. 1b) exhibited coarser and more rugged surface than the raw petroleum coke and even regular AC (Fig. 1a and c), as will be shown in XRD and BET results. This coarser surface can increase the specific surface area of the PC, which is beneficial for the formation of active sites for oxygen reduction reaction. The formation of porous structure and the increase in roughness can be attributed to the removal of inorganic impurities, as will be shown in the XPS measurements.
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samples. The parameters d002 (crystal plane spacing) and D (microcrystalline thickness) for the (002) and (100) peaks were calculated by DebyeeScherer equations [30]. It can be found that D increased from 0.824 nm for PC sample to 1.628 nm for AC sample, indicating that the surface of PC sample was more disordered than that of AC. A value of d002 0.321 nm for PC sample was slightly smaller than d002 0.337 nm for AC sample. It was acknowledged that pore walls in the activated carbon were made up of graphitic microcrystallites [26] and hence a decrease in crystallites thickness led to the enhancement of the pores. As compared to the AC sample, the microcrystalline thickness D for PC was smaller, indicating the development of pores. According to the N2 adsorption and desorption isotherms in Fig. 2b, the isotherm of PC shifted to a higher range of volume, while the AC sample shifted to a lower direction. The results of BET surface area analysis of PC and AC samples are listed in Table 2. The BET specific surface area and the pore volume of the PC sample were 2213.45 m2 g1 and 1.13 cm3 g1, respectively, whereas the AC sample's BET specific surface area and pore volume were 2132.95 m2 g1 and 1.08 cm3 g1. The BET surface area of the PC sample was 1.04 times that of the AC sample, which was mainly due to the increase of micropore area. The micropore area of PC was 1.37 times that of the AC sample, while the mesopore area of PC was just 73.59% of the latter. It has been reported that the performance of carbonaceous material cathode was due to the high surface area of the material [31], thus, the larger surface area of PC could enhance the electrocatalytic performance of cathodes. Precisely, the higher amount of micropores, the most possible site of 4-electron oxygen reduction reaction [16], should be the main positive factor for the high performance of PC sample.
Surface chemical characterizations of PC materials The porous nature of PC material The porous nature of PC material was investigated by N2 adsorption analyzer and XRD measurement. The XRD patterns of PC and AC material are shown in Fig. 2a. There were two slightly broad diffraction peaks around 2q ¼ 26 and 43 in the two spectrums, corresponding to the diffraction of (002) and (100), respectively. The (002) and (100) peak of sample AC was sharper than PC, suggesting that the surface arrangement of the PC was more disordered than AC sample. Table 1 shows the microcrystalline structural parameters of the PC and AC
The XPS measurement was performed to determine the composition of the PC and AC samples, and results are shown in Fig. 3a and b, respectively. The relative element contents on the sample surface is given in Table 3. The AC sample was only composed of C and O, while other elements were found in PC, such as S, P, Al, Cu, Ca and Zn. These elements may be the residue of the activation process of petroleum coke, which had complex components. In AC sample, the largest proportion of the sample was C (93.93%) and the proportion of O was just 6.07%. On the contrary, the proportion of C in the PC
Fig. 1 e SEM images of the surface of the raw petroleum coke (a), PC sample (b) and the AC sample (c). Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Table 2 e BET analysis of PC and AC samples. BET surface area (m2/g) Micropore area (m2/g) Mesopore area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Mesopore volume (cm3/g)
PC
AC
2213.45 1386.13 827.33 1.13 0.65 0.48
2132.95 1008.64 1124.31 1.08 0.45 0.61
sample decreased to 64.60%, and other organic elements (P and S) and metal elements (Al, Cu, Ca and Zn) appeared. The majority of the O 1s signals lied between 530 and 536 eV (Fig. 3c) and in this region a multitude of oxygen containing functional groups had their BE. It had been reported that a shift of the O 1s signal to lower BEs may indicate an increase in functional groups of carboxylic acid type and a decrease in groups such as hydroquinones and ethers [32]. According to Fig. 3c, the O 1s signal of the PC sample was shifted from 532.479 eV to lower BEs (532.219 eV), which may indicate more functional groups of carboxylic acid existed on the PC sample than AC sample. The C 1s signals of the two samples are shown in Fig. 3d. Both of the sample consisted of a major graphitic carbon peak (peak 1 at 284.5 eV) and a low peak (peak 2 around 286 eV), which was assigned to carbon associated with oxygen [32]. This result also confirms the existence of oxygen containing groups on the samples, as will be shown in the next FTIR measurements. Fig. 2c shows the FTIR spectra of the PC sample and the AC sample. Both the two samples exhibited noticeable IR bands at around 3500 cm1 and between 1000 and 1700 cm1. The broad band at around 3500 cm1 can be assigned to the OeH stretching vibration mode of hydroxyl functional groups. The wide peaks between 800 and 1300 cm1 may be assigned to the bond between carbon and oxygen in functional groups containing oxygen [33], which is consistent with the O 1s XPS results. This peak of PC was deeper than that of AC sample, indicating that the content of functional groups containing oxygen in PC sample was more than that of the AC sample. This result was in accord with the XPS result that the content of O in PC (11.49, at %) was higher than that of the AC sample.
The catalytic ORR activity of PC air cathode
Fig. 2 e X-ray diffraction patterns (a), N2 adsorption and desorption isotherms (b) and FTIR spectra (c) of the PC and AC samples.
Table 1 e Microcrystalline structural parameters of the PC and AC samples. Sample
2q of (100)
2q of (002)
FWHM
D (nm)
d002 (nm)
d100 (nm)
PC AC
43.360 42.760
27.720 26.440
0.188 0.094
0.824 1.628
0.321 0.337
0.209 0.211
The electrocatalyst ORR activity performance of PC air cathode was evaluated in a three-electrode system by using LSV in PBS solution (Fig. 4). The Eonset of PC cathode (0.199 ± 0.005 V) was higher than that of the AC cathode (0.189 ± 0.004 V), indicating that PC cathode had a lower overpotential than AC cathode. The Eonset of AC cathode was a little higher than the result reported by Dong Heng [9], which was likely due to the use of different type AC in the tests. Fig. 4 also shows that PC cathode had a significantly higher current density than the AC cathode. The current density of PC at 0.1 V was 0.936 ± 0.023 mA m2, which was 37% higher than that of the AC cathode (0.683 ± 0.031 mA m2). The high catalytic activity of PC was supposedly due to its morphological structure and active surface groups, as discussed before. The Nyquist plots of PC and AC air cathodes measured by electrochemical
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 3 e XPS survey spectra of the PC sample (a) and the AC sample (b), high-resolution O 1s (c) and C 1s (d) spectra of the PC and AC samples.
Table 3 e Elements analysis and related properties of the PC and AC samples. Sample
PC AC
Content, At% O
C
S
P
Al
Cu
Ca
Zn
13.49 6.07
78.60 93.93
1.73 0
1.84 0
1.65 0
1.50 0
1.45 0
1.35 0
impedance spectroscopy (EIS) are shown in Fig. 5a. An equivalent circuit for porous electrode was used to investigate the electrochemical properties [34]. The total resistances of three different air cathodes were compared in Fig. 5b. The
Fig. 4 e Line sweep voltammograms obtained in PBS solution for PC and AC air cathodes, respectively, at a scan rate of 0.1 mV s¡1.
total resistance of PC (14.75 U) was lower than that of AC (16.34 U). Since the ohmic resistances (Ro) and diffusion resistances (Rd) were similar for these two kinds of cathode, charge transfer resistance (Rct) led to the most of differences in the total internal resistance. Utilization of PC led to a decrease in Rct, with value decreased from 2.14 U (AC) to 0.83 U (PC), which indicating that the PC material was better for the charge transfer in the catalyst layer than AC.
MFC performance of PC air cathode The PC and AC air cathodes were constructed in singlechamber MFCs and the performance was evaluated by polarization and power density curves in a week as shown in Fig. 6a and b. The power density achieved by AC air cathode (833.53 ± 50.34 mW m2) is comparable to that reported by other papers [9,16,35,36]. The PC air-cathode MFC had the maximum power density of 1029.77 ± 99.53 mW m2, which was 23.52% higher than that of the AC air-cathode MFC (Fig. 6a) and Pt/C air-cathode MFC (Fig. S1 in Supplementary Material). The open circuit voltage (OCV) of the PC aircathode MFC (664.67 mV) was also higher than that of AC air-cathode MFC (620.33 mV). The potential curves (Fig. 6b) show that the improved power output came from the difference of the cathodes. To examine the long-term stability of PC air cathodes, the maximum power densities were also measured after 1 month of operation. MFCs with PC air cathodes still showed higher power densities, with a maximum of 931.25 ± 45.26 mW m2 (Fig. 7). This was 16.27% higher than that with the normal AC air cathodes (798.48 ± 32.15 mW m2). This result indicated PC air cathodes had comparable durability to AC air cathodes.
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 6 e Polarization and power density curves (a) and electrode potential (b) of MFCs equipped with PC cathode and AC cathode with Ag/AgCl as the reference electrode. Mean values and error bars are based on 3 time repeats. Fig. 5 e Nyquist plots of EIS by PC and AC air cathodes (a) and component analysis of internal resistance for different cathodes (b). Lines marked as ‘Cal’ were fitting data from the equivalent circuit.
The carbonaceous material from petroleum coke was featured with high surface area and naturally doped with oxygen containing groups and the trace transition metal elements. The carboxylic acid groups could increase the hydrophilicity of carbon surface so that the catalytic active sites are more accessible to dissolved oxygen (together with electrolyte) [20,37]. Therefore, the PC sample may have the faster transport of reactants for ORR. The transition metal in carbonbased catalysts could also enhance the ORR activity of the cathodes [38]. Thus, we proposed that the higher onset potential and current density of the PC cathode due to the more content of oxygen containing groups and the trace transition metal elements doped in the PC sample (Fig. 8). These “active sites” [39,40] could have a stimulating effect on the cathode catalyst performance, and their synergistic effect on the power production requires further investigation. These results demonstrated that PC is an efficient and cost-effective cathode catalyst for practical MFC applications. It should be also mentioned that although the PC air cathode developed in
Fig. 7 e The maximum power densities of the MFCs with PC or AC air cathode measured in a week and after 1 month.
this work is promising in term of its higher performance with lower cost than AC and Pt/C air cathodes, some other issues, such as stability over long time operation and optimization of preparation process merit further research.
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 8 e Schematic of the petrocoke carbon and its structural and chemical properties facilitating the oxygen reduction reaction.
Conclusion We proposed a convenient, environmental and economical route in this work to prepare carbonaceous materials from petroleum coke. The prepared carbonaceous materials were featured with a high surface area and naturally doped with oxygen containing groups and the trace transition metal elements as active sites. The petroleum coke is abundant waste from oil refining industry. In addition, the feasibility of using PC as aircathode catalyst in MFC was investigated. MFCs with PC cathodes showed high performance and good durability. It is supposed that the high catalytic activity of PC is mainly due to its large surface area and surface morphologies. A large number of oxygen containing functional groups and a small quantity of residue elements were found on the surface of PC sample, which may also contributes to the catalytic activity. The reuse of petroleum coke would not only reduce waste, but also contribute significantly to the practical application of MFC technology.
Acknowledgments This work was partially supported by the Natural Science Foundation of Tianjin City (No. 15JCYBJC21400).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.08.025
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Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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[19] Santoro C, Babanova S, Artyushkova K, Atanassov P, Greenman J, Cristiani P, et al. The effects of wastewater types on power generation and phosphorus removal of microbial fuel cells (MFCs) with activated carbon (AC) cathodes. Int J Hydrogen Energy 2014;39:21796e802. [20] Santoro C, Artyushkova K, Babanova S, Atanassov P, Ieropoulos I, Grattieri M, et al. Parameters characterization and optimization of activated carbon (AC) cathodes for microbial fuel cell application. Bioresour Technol 2014;163:54e63. [21] Deng Q, Li X, Zuo J, Ling A, Logan BE. Power generation using an activated carbon fiber felt cathode in an upflow microbial fuel cell. J Power Sources 2010;195:1130e5. [22] Wen Q, Wang SY, Yan J, Cong LJ, Chen Y, Xi HY. Porous nitrogen-doped carbon nanosheet on graphene as metal-free catalyst for oxygen reduction reaction in air-cathode microbial fuel cells. Bioelectrochemistry 2014;95:23e8. [23] Zheng Y, Wang X, Cui L. Synthesis of ZnS from organic sulfur in petroleum coke and its photocatalysis properties. Petroleum Sci 2010;7:268e72. [24] Nemanova V, Abedini A, Liliedahl T, Engvall K. Co-gasification of petroleum coke and biomass. Fuel 2013;117:870e5. [25] Otowa T, Nojima Y, Miyazaki T. Development of KOH activated high surface area carbon and its application to drinking water purification. Carbon 1997;35:1315e9. [26] Chunlan L, Shaoping X, Yixiong G, Shuqin L, Changhou L. Effect of pre-carbonization of petroleum cokes on chemical activation process with KOH. Carbon 2005;43:2295e301. D, Lillo-Ro denas MA, Cazorla-Amoro s D, [27] Lozano-Castello Linares-Solano A. Preparation of activated carbons from Spanish anthracite: I. Activation by KOH. Carbon 2001;39:741e9. [28] Zhang P, Li KX, Liu XH. Carnation-like MnO2 modified activated carbon air cathode improve power generation in microbial fuel cells. J Power Sources 2014;264:248e53. [29] Bretschger O, Obraztsova A, Sturm CA, Chang IS, Gorby YA, Reed SB, et al. Current production and metal oxide reduction by Shewanella oneidensis MR-1 wild type and mutants. Appl Environ Microbiol 2007;73:7003e12. [30] Anandan K, Rajendran V. Rose-like SnO2 nanostructures synthesized via facile solvothermal technique and their optical properties. J Phys Sci 2010;14:227e34.
[31] Zhang F, Cheng S, Pant D, Bogaert GV, Logan BE. Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell. Electrochem Commun 2009;11:2177e9. [32] Jansen R, Van Bekkum H. XPS of nitrogen-containing functional groups on activated carbon. Carbon 1995;33:1021e7. [33] Terzyk AP. The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: Part II. TG, FTIR, and XPS analysis of carbons and the temperature dependence of adsorption kinetics at the neutral pH. Colloid Surface A 2001;177:23e45. [34] Cheng SA, Liu WF, Guo J, Sun D, Pan B, Ye YL, et al. Effects of hydraulic pressure on the performance of single chamber air-cathode microbial fuel cells. Biosens Bioelectron 2014;56:264e70. [35] Dong H, Yu H, Yu H, Gao N, Wang X. Enhanced performance of activated carbonepolytetrafluoroethylene air-cathode by avoidance of sintering on catalyst layer in microbial fuel cells. J Power Sources 2013;232:132e8. [36] Cheng S, Wu J. Air-cathode preparation with activated carbon as catalyst, PTFE as binder and nickel foam as current collector for microbial fuel cells. Bioelectrochemistry 2013;92:22e6. [37] Lee JS, Park GS, Kim ST, Liu M, Cho J. A highly efficient electrocatalyst for the oxygen reduction reaction: N-doped ketjenblack incorporated into Fe/Fe3C-functionalized melamine foam. Angew Chem 2013;125:1060e4. [38] Tributsch H, Koslowski UI, Dorbandt I. Experimental and theoretical modeling of Fe-, Co-, Cu-, Mn-based electrocatalysts for oxygen reduction. Electrochim Acta 2008;53:2198e209. [39] Kobayashi M, Niwa H, Harada Y, Horiba K, Oshima M, Ofuchi H, et al. Role of residual transition-metal atoms in oxygen reduction reaction in cobalt phthalocyanine-based carbon cathode catalysts for polymer electrolyte fuel cell. J Power Sources 2011;196:8346e51. [40] Grumelli D, Wurster B, Stepanow S, Kern K. Bio-inspired nanocatalysts for the oxygen reduction reaction. Nat Commun 2013;4:2904.
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells Peng Zhang a,b, Xian-Hua Liu a,*, Ke-Xun Li b,**, Yi-Ren Lu a a
Tianjin Key Lab. of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072, PR China b College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, PR China
article info
abstract
Article history:
Petrocoke carbon (PC) was prepared by pyrolysis of easily available byproduct of oil
Received 1 May 2015
refining, petroleum coke. Thus-obtained PC not only possesses highly porous surface area,
Received in revised form
but also contains heteroatoms such as phosphate and sulfur in its framework. These
12 July 2015
properties make petrocoke carbon a good choice as a precursor for high performance air-
Accepted 12 August 2015
cathode catalyst. Microbial fuel cells (MFCs) with PC as air-cathode catalyst produced a
Available online xxx
maximum power density of 1029.77 ± 99.53 mW m2, which was higher than those with Pt/ C cathodes or activated carbon cathodes. Furthermore, PC cathodes have comparable
Keywords:
durability to activated carbon cathodes, demonstrating that the PC can be a low-cost and
Petroleum coke
reliable alternative to precious-metal-based electrocatalysts for the air cathode in MFCs.
Air cathode
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst MFCs
Introduction In the last decade, the application of microbial fuel cells (MFCs) has made tremendous progress and the MFC performance has increased remarkably [1e5]. For the scale up of MFCs, air cathode is the most useful configuration because of the inexhaustible availability of air and high redox potential of oxygen reduction [6]. During operation, the air passes through the cathode to the interior cathode active surface in contact with the cell's electrolyte. Catalysts are needed to enhance the oxygen reduction reaction (ORR) at the cathode. To date, platinum (Pt) has been the most used cathode catalyst in fuel cells. However, Pt based catalysts have a high cost and are easily poisoned by intermediates, which critically impeded
the extensive application of MFCs [7]. Recently, carbonaceous materials such as carbon nanotubes, graphene, and activated carbon (AC) had been found to be effective for the ORR [8e14] and were widely used to replace the precious metal catalysts. The excellent performance of carbonaceous materials was deemed to its large surface area, which reduced the overpotential with small local current densities [15], excellent ORR electrocatalytic performance, and long-term durability [10]. Meanwhile, the lower price of the carbonaceous materials enhanced the application of MFC technology in practice. Many kinds of carbonaceous materials had been used as MFC cathode material and showed different rates of performance [14,16e20]. Activated carbon fiber felt cathode has been reported being applied in an upflow MFC and a maximum power density of 784 mW m2 was achieved [21]. Carbonaceous
* Corresponding author. Tel./fax: þ86 22 27402367. ** Corresponding author. Tel./fax: þ86 22 23495200. E-mail addresses: [email protected] (X.-H. Liu), [email protected] (K.-X. Li). http://dx.doi.org/10.1016/j.ijhydene.2015.08.025 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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materials doped with heteroatoms such as nitrogen, sulfur, phosphorus, and boron have also been investigated because of their outstanding ORR catalytic performances [10]. Fox example, nitrogen-doped nanosheet on graphene was applied as the cathode catalyst in MFC and a peak power density of 1159.34 mW m2 was obtained [22]. However, most synthetic approaches of these carbonaceous materials are usually complex and costly, which limited their application in fuel cells. Petroleum coke is a waste byproduct of oil refining and is very abundant and cheap [23]. It is a black solid material consisting of polycyclic aromatic hydrocarbons with low hydrogen content and usually composed of C: 91e99.5%, H: 0.035e4%, S: 0.5e8%, (N þ O): 1.3e3.8% and the rest is metals [24]. Effort has been directed towards petroleum coke utilization in the fields of combustion and gasification to generate electric power or produce syngas. However, the applications for petroleum coke have been limited due to the high sulfur content (~6 wt%) and low surface area (<5 m2 g1). It had been reported that the activated petroleum coke can have a greater specific area than the ordinary activated carbon [25]. Due to its lower cost and higher specific area, PC may be considered as a high-performance and cost-effective cathode material. Until now, there is still little information available on its application in MFCs. In this study, we present an economical and environmental route to synthesize carbonaceous materials with highly porous surface area and active sites using a simple KOH activation process. These carbonaceous materials were utilized to synthesize air cathodes by a rolling method [9]. The performance of these materials as air-cathode catalyst for oxygen reduction reaction was evaluated in electrochemical tests and MFC systems. In addition, the surface properties of the PC were examined by scanning electron microscopy (SEM), BrunauereEmmetteTeller analysis (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). Furthermore, we examined the stability of these PC cathodes and compared them to normal AC cathodes after 1 month of continuous operation.
Materials and methods Preparation of PC cathode Petroleum coke from SINOPEC Tianjin Company was used as raw material and smashed by wet ball mill and dried in an oven at 80 C for 12 h. Then the petroleum coke was mixed with KOH with a weight ratio of 1:4 (petroleum coke: KOH). KOH is one of the typical alkaline metal compounds that is widely used for the chemical activation of coal precursor, chars and petroleum coke [26]. The ratio of petroleum coke to KOH plays an important role in the development of carbon porosity. According to D. Lozano-Castello [27], surface area and micropore volume reach the maximum with KOH/C ratio of 4:1. The mixture was ground with a mortar to ensure that the compositions could be well distributed, and it was activated in a pipe furnace in nitrogen atmosphere (purity: 99.99%) at 800 C. The activation process was that: the
temperature was increased to 300 C at a speed of 2 C min1 from room temperature and kept at this temperature for 1 h, then the temperature increased to the activation temperature (800 C) at a speed of 5 C min1 and this activation process lasted for 1 h. The activation product was cooled in nitrogen and washed with distilled water until the pH approached 7. Then the product was dried in the oven at 120 C for 1 day. The product was ground again to form a state of powder, which was used in the construction of PC cathode. The PC cathode was constructed with the same procedure as AC cathode described by Dong [9]: the PC was mixed with polytetrafluoroethylene (PTFE) and rolled on the water facing surface of stainless steel mesh with the mixture of carbon black and PTFE sintered on the air facing side. The normal AC (2132.95 m2 g1, Yihuan Carbon Co. Ltd., Fujian, China) was also used as the catalyst to construct AC cathode by the same method.
MFC setup and operation Single-chamber MFCs (40-mm long cylindrical chamber; volume 28 mL) were constructed as previously described [28]. Anodes were round-shape carbon felts (diameter 30 mm; thickness 5 mm). Both the anode and cathode had the same projected area of 7 cm2 and were connected to an external resistance of 1000 U using titanium wires. To ensure the same experimental conditions, the different cathodes (PC and AC) were constructed in the same MFC devices. The MFCs were inoculated using wastewater from the Jizhuangzi municipal sewage treatment plant (Tianjin, China) and had been cultivated for 2 months at 30 C in a biochemical incubator. The medium was 1 g L1 sodium acetate with vitamins and minerals dissolved in phosphate buffer solution (PBS) [29].
Measurements and analysis A multi-channel voltage logger (MPS-10, Morpheus Electronic Co. Ltd, Beijing, China) was used to record the voltages. To obtain polarization curves and power density curves, a variable resistance box (Tianjin, China) was used to vary the external resistance from 9000 U to 50 U. The voltage was recorded after a fixed time (about 30 min) to ensure a stable state. Cathode and anode potentials during polarization were measured using an Ag/AgCl electrode as the reference electrode. Tests were conducted in the first week and after 1 month of operation. All experiments were conducted in triplicate. Linear sweep voltammetry (LSV) of all the cathodes were measured at a scan rate of 0.1 mV s1 from open circuit potential (OCP) to 0.3 V by a CHI 660 potentiostat (Shanghai, China). An Ag/AgCl electrode was used as reference electrode and platinum sheet (1 cm2) was used as counter electrode. Electrochemical impedance spectroscopy (EIS) of all the cathodes was performed over a frequency range of 100 kHze100 mHz using a potentiostat (VersaSTAT 3, Princeton Applied Research, USA). An FTIR spectrometer (FTS6000) was utilized to detect the IR-observable functional groups on the surface of the PC and normal AC in the wavenumber range of 400e4000 cm1. Small amounts of samples were mixed with KBr and pressed into
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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pellets, which contains about 0.5% sample in weight [26]. To characterize the crystal structure of the catalysts, a Rigaku D/ Max-2500 V X-ray diffractometer was used to measure the Xray intensity over a diffraction 2-theta angle from 5 to 80. The dimension of carbon crystallites was analyzed by X-ray diffraction line broadening. The layer dimension D perpendicular to the basal plane was calculated by the (002) reflection. XPS (Axis Ultra DLD, Kratos Analytical Ltd.) was employed to obtain information about the elemental composition of the PC and AC samples. CasaXPS software was used to process the XPS data for the elemental analysis. The morphology of the PC and AC surface were examined by SEM (Hitachi S-3500N, Tokyo, Japan) at an accelerating voltage of 20 kV. The BET surface area of the catalyst layer was measured on an ASAP 2020/TriStar 3000 (Micromeritics) using nitrogen as adsorbents.
Results and discussion Surface morphology of PC Fig. 1 shows the surface morphology of raw petroleum coke, PC, and AC. The surface of raw petroleum coke (Fig. 1a) was smooth and wrinkled. After activation, the surface of PC (Fig. 1b) exhibited coarser and more rugged surface than the raw petroleum coke and even regular AC (Fig. 1a and c), as will be shown in XRD and BET results. This coarser surface can increase the specific surface area of the PC, which is beneficial for the formation of active sites for oxygen reduction reaction. The formation of porous structure and the increase in roughness can be attributed to the removal of inorganic impurities, as will be shown in the XPS measurements.
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samples. The parameters d002 (crystal plane spacing) and D (microcrystalline thickness) for the (002) and (100) peaks were calculated by DebyeeScherer equations [30]. It can be found that D increased from 0.824 nm for PC sample to 1.628 nm for AC sample, indicating that the surface of PC sample was more disordered than that of AC. A value of d002 0.321 nm for PC sample was slightly smaller than d002 0.337 nm for AC sample. It was acknowledged that pore walls in the activated carbon were made up of graphitic microcrystallites [26] and hence a decrease in crystallites thickness led to the enhancement of the pores. As compared to the AC sample, the microcrystalline thickness D for PC was smaller, indicating the development of pores. According to the N2 adsorption and desorption isotherms in Fig. 2b, the isotherm of PC shifted to a higher range of volume, while the AC sample shifted to a lower direction. The results of BET surface area analysis of PC and AC samples are listed in Table 2. The BET specific surface area and the pore volume of the PC sample were 2213.45 m2 g1 and 1.13 cm3 g1, respectively, whereas the AC sample's BET specific surface area and pore volume were 2132.95 m2 g1 and 1.08 cm3 g1. The BET surface area of the PC sample was 1.04 times that of the AC sample, which was mainly due to the increase of micropore area. The micropore area of PC was 1.37 times that of the AC sample, while the mesopore area of PC was just 73.59% of the latter. It has been reported that the performance of carbonaceous material cathode was due to the high surface area of the material [31], thus, the larger surface area of PC could enhance the electrocatalytic performance of cathodes. Precisely, the higher amount of micropores, the most possible site of 4-electron oxygen reduction reaction [16], should be the main positive factor for the high performance of PC sample.
Surface chemical characterizations of PC materials The porous nature of PC material The porous nature of PC material was investigated by N2 adsorption analyzer and XRD measurement. The XRD patterns of PC and AC material are shown in Fig. 2a. There were two slightly broad diffraction peaks around 2q ¼ 26 and 43 in the two spectrums, corresponding to the diffraction of (002) and (100), respectively. The (002) and (100) peak of sample AC was sharper than PC, suggesting that the surface arrangement of the PC was more disordered than AC sample. Table 1 shows the microcrystalline structural parameters of the PC and AC
The XPS measurement was performed to determine the composition of the PC and AC samples, and results are shown in Fig. 3a and b, respectively. The relative element contents on the sample surface is given in Table 3. The AC sample was only composed of C and O, while other elements were found in PC, such as S, P, Al, Cu, Ca and Zn. These elements may be the residue of the activation process of petroleum coke, which had complex components. In AC sample, the largest proportion of the sample was C (93.93%) and the proportion of O was just 6.07%. On the contrary, the proportion of C in the PC
Fig. 1 e SEM images of the surface of the raw petroleum coke (a), PC sample (b) and the AC sample (c). Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Table 2 e BET analysis of PC and AC samples. BET surface area (m2/g) Micropore area (m2/g) Mesopore area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Mesopore volume (cm3/g)
PC
AC
2213.45 1386.13 827.33 1.13 0.65 0.48
2132.95 1008.64 1124.31 1.08 0.45 0.61
sample decreased to 64.60%, and other organic elements (P and S) and metal elements (Al, Cu, Ca and Zn) appeared. The majority of the O 1s signals lied between 530 and 536 eV (Fig. 3c) and in this region a multitude of oxygen containing functional groups had their BE. It had been reported that a shift of the O 1s signal to lower BEs may indicate an increase in functional groups of carboxylic acid type and a decrease in groups such as hydroquinones and ethers [32]. According to Fig. 3c, the O 1s signal of the PC sample was shifted from 532.479 eV to lower BEs (532.219 eV), which may indicate more functional groups of carboxylic acid existed on the PC sample than AC sample. The C 1s signals of the two samples are shown in Fig. 3d. Both of the sample consisted of a major graphitic carbon peak (peak 1 at 284.5 eV) and a low peak (peak 2 around 286 eV), which was assigned to carbon associated with oxygen [32]. This result also confirms the existence of oxygen containing groups on the samples, as will be shown in the next FTIR measurements. Fig. 2c shows the FTIR spectra of the PC sample and the AC sample. Both the two samples exhibited noticeable IR bands at around 3500 cm1 and between 1000 and 1700 cm1. The broad band at around 3500 cm1 can be assigned to the OeH stretching vibration mode of hydroxyl functional groups. The wide peaks between 800 and 1300 cm1 may be assigned to the bond between carbon and oxygen in functional groups containing oxygen [33], which is consistent with the O 1s XPS results. This peak of PC was deeper than that of AC sample, indicating that the content of functional groups containing oxygen in PC sample was more than that of the AC sample. This result was in accord with the XPS result that the content of O in PC (11.49, at %) was higher than that of the AC sample.
The catalytic ORR activity of PC air cathode
Fig. 2 e X-ray diffraction patterns (a), N2 adsorption and desorption isotherms (b) and FTIR spectra (c) of the PC and AC samples.
Table 1 e Microcrystalline structural parameters of the PC and AC samples. Sample
2q of (100)
2q of (002)
FWHM
D (nm)
d002 (nm)
d100 (nm)
PC AC
43.360 42.760
27.720 26.440
0.188 0.094
0.824 1.628
0.321 0.337
0.209 0.211
The electrocatalyst ORR activity performance of PC air cathode was evaluated in a three-electrode system by using LSV in PBS solution (Fig. 4). The Eonset of PC cathode (0.199 ± 0.005 V) was higher than that of the AC cathode (0.189 ± 0.004 V), indicating that PC cathode had a lower overpotential than AC cathode. The Eonset of AC cathode was a little higher than the result reported by Dong Heng [9], which was likely due to the use of different type AC in the tests. Fig. 4 also shows that PC cathode had a significantly higher current density than the AC cathode. The current density of PC at 0.1 V was 0.936 ± 0.023 mA m2, which was 37% higher than that of the AC cathode (0.683 ± 0.031 mA m2). The high catalytic activity of PC was supposedly due to its morphological structure and active surface groups, as discussed before. The Nyquist plots of PC and AC air cathodes measured by electrochemical
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 3 e XPS survey spectra of the PC sample (a) and the AC sample (b), high-resolution O 1s (c) and C 1s (d) spectra of the PC and AC samples.
Table 3 e Elements analysis and related properties of the PC and AC samples. Sample
PC AC
Content, At% O
C
S
P
Al
Cu
Ca
Zn
13.49 6.07
78.60 93.93
1.73 0
1.84 0
1.65 0
1.50 0
1.45 0
1.35 0
impedance spectroscopy (EIS) are shown in Fig. 5a. An equivalent circuit for porous electrode was used to investigate the electrochemical properties [34]. The total resistances of three different air cathodes were compared in Fig. 5b. The
Fig. 4 e Line sweep voltammograms obtained in PBS solution for PC and AC air cathodes, respectively, at a scan rate of 0.1 mV s¡1.
total resistance of PC (14.75 U) was lower than that of AC (16.34 U). Since the ohmic resistances (Ro) and diffusion resistances (Rd) were similar for these two kinds of cathode, charge transfer resistance (Rct) led to the most of differences in the total internal resistance. Utilization of PC led to a decrease in Rct, with value decreased from 2.14 U (AC) to 0.83 U (PC), which indicating that the PC material was better for the charge transfer in the catalyst layer than AC.
MFC performance of PC air cathode The PC and AC air cathodes were constructed in singlechamber MFCs and the performance was evaluated by polarization and power density curves in a week as shown in Fig. 6a and b. The power density achieved by AC air cathode (833.53 ± 50.34 mW m2) is comparable to that reported by other papers [9,16,35,36]. The PC air-cathode MFC had the maximum power density of 1029.77 ± 99.53 mW m2, which was 23.52% higher than that of the AC air-cathode MFC (Fig. 6a) and Pt/C air-cathode MFC (Fig. S1 in Supplementary Material). The open circuit voltage (OCV) of the PC aircathode MFC (664.67 mV) was also higher than that of AC air-cathode MFC (620.33 mV). The potential curves (Fig. 6b) show that the improved power output came from the difference of the cathodes. To examine the long-term stability of PC air cathodes, the maximum power densities were also measured after 1 month of operation. MFCs with PC air cathodes still showed higher power densities, with a maximum of 931.25 ± 45.26 mW m2 (Fig. 7). This was 16.27% higher than that with the normal AC air cathodes (798.48 ± 32.15 mW m2). This result indicated PC air cathodes had comparable durability to AC air cathodes.
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 6 e Polarization and power density curves (a) and electrode potential (b) of MFCs equipped with PC cathode and AC cathode with Ag/AgCl as the reference electrode. Mean values and error bars are based on 3 time repeats. Fig. 5 e Nyquist plots of EIS by PC and AC air cathodes (a) and component analysis of internal resistance for different cathodes (b). Lines marked as ‘Cal’ were fitting data from the equivalent circuit.
The carbonaceous material from petroleum coke was featured with high surface area and naturally doped with oxygen containing groups and the trace transition metal elements. The carboxylic acid groups could increase the hydrophilicity of carbon surface so that the catalytic active sites are more accessible to dissolved oxygen (together with electrolyte) [20,37]. Therefore, the PC sample may have the faster transport of reactants for ORR. The transition metal in carbonbased catalysts could also enhance the ORR activity of the cathodes [38]. Thus, we proposed that the higher onset potential and current density of the PC cathode due to the more content of oxygen containing groups and the trace transition metal elements doped in the PC sample (Fig. 8). These “active sites” [39,40] could have a stimulating effect on the cathode catalyst performance, and their synergistic effect on the power production requires further investigation. These results demonstrated that PC is an efficient and cost-effective cathode catalyst for practical MFC applications. It should be also mentioned that although the PC air cathode developed in
Fig. 7 e The maximum power densities of the MFCs with PC or AC air cathode measured in a week and after 1 month.
this work is promising in term of its higher performance with lower cost than AC and Pt/C air cathodes, some other issues, such as stability over long time operation and optimization of preparation process merit further research.
Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Fig. 8 e Schematic of the petrocoke carbon and its structural and chemical properties facilitating the oxygen reduction reaction.
Conclusion We proposed a convenient, environmental and economical route in this work to prepare carbonaceous materials from petroleum coke. The prepared carbonaceous materials were featured with a high surface area and naturally doped with oxygen containing groups and the trace transition metal elements as active sites. The petroleum coke is abundant waste from oil refining industry. In addition, the feasibility of using PC as aircathode catalyst in MFC was investigated. MFCs with PC cathodes showed high performance and good durability. It is supposed that the high catalytic activity of PC is mainly due to its large surface area and surface morphologies. A large number of oxygen containing functional groups and a small quantity of residue elements were found on the surface of PC sample, which may also contributes to the catalytic activity. The reuse of petroleum coke would not only reduce waste, but also contribute significantly to the practical application of MFC technology.
Acknowledgments This work was partially supported by the Natural Science Foundation of Tianjin City (No. 15JCYBJC21400).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.08.025
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Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025
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Please cite this article in press as: Zhang P, et al., Heteroatom-doped highly porous carbon derived from petroleum coke as efficient cathode catalyst for microbial fuel cells, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.025